This invention relates to oscillator circuits for use in integrated circuits.
One of the most frequently-used analog circuits in today""s IC""s is the Pierce-type two-pin oscillator. Typically, this type of oscillator is used to generate clock signals in microprocessors and other integrated systems.
Such an oscillator is typically formed by a resonator embedded between two capacitors connected to ground and between the input and output of an inverter. FIG. 1 shows the schematic circuit of such an oscillator.
This oscillator is of simple construction, but presents several disadvantages:
The amplitude at the output is limited by non-linearities generating harmonics which may cause electromagnetic coupling (EMC) problems.
The amplitude across the resonator is high, resulting in high power dissipation in the resonator, limiting its long-term frequency stability.
Since both transistors constituting the inverter conduct simultaneously during a large portion of the period of oscillation, the oscillator has high current consumption.
A high impedance current path is needed between the input and output of the inverter to ensure its correct biasing. Leakage of external components connected to this path may alter operating conditions of the oscillator.
Several solutions have been proposed to deal with these disadvantages. FIG. 2 shows a prior art circuit known from U.S. Pat. No. 4,405,906 which avoids the above mentioned drawbacks by regulation of the oscillation amplitude. In this known circuit two DC bias current sources polarize the driver transistors, whereas the currents from these current sources are sunk to the supply rails across transistors connected between the driver transistors common drain and their gate.
This known circuit presents two important drawbacks. Firstly, well-controlled DC current sources are needed to polarize correctly the driver transistors. Secondly, a start-up circuit is needed to ensure that both driver transistors remain in saturation when the circuit is powered on.
From U.S. Pat. No. 3,902,141 there is known a quartz oscillator circuit having an amplifier formed by two complementary semiconductors whose sources, gates and drains are connected in parallel in an alternate arrangement with a power supply, a quartz crystal connected between the drains and the gates, a detection capacitor connected between the gates and the sources and a charge capacitor connected between the sources and the drains. An integration capacitor is connected between the semiconductors"" gates so as to control the class of operation of the amplifier in dependence on the oscillation amplitude.
A very low current Pierce oscillator is known from U.S. Pat. No. 4,360,789.
It is an object of the present invention to provide an oscillator circuit wherein the above-mentioned disadvantages may be overcome or at least alleviated.
In accordance with a first aspect of the invention there is provided an oscillator circuit as claimed in claim 1.
In accordance with a second aspect of the invention there is provided an oscillator circuit as claimed in claim 11.